Many polymeric materials are hydrophobic. When such materials are formed into films, beads, membranes or the like, their hydrophobic nature prevents or inhibits "wetting" by water.
When used to describe a surface, the term "hydrophobic" means that water on that surface has a contact angle of greater than ninety degrees. By contrast, the term "hydrophilic" applies to those polymeric surfaces which have a contact angle of less than ninety degrees.
While hydrophobic materials are well known in the art and easily prepared, their usefulness in many processes and products is severely restricted by their hydrophobicity. There have been numerous prior attempts to render a hydrophobic material hydrophilic in order to be useful in processes where water is present and must "wet" the surface of the material.
Several efforts have concentrated in rendering hydrophilic a porous hydrophobic polymeric membrane. Despite the low cost of preparation of such hydrophobic materials in the form of porous membranes, such membranes are not useful as membranes in aqueous systems because capillary forces at the pores of such hydrophobic materials prevent the wetting of the pores by water, aqueous solutions, or other high surface tension organic solutions.
Treatment of the surfaces of hydrophobic materials, such as porous membranes, made from polyolefins has been attempted using surfactant coatings such as the silicone glycol copolymer disclosed in U.S. Pat. No. 3,853,601 (Taskier) or the nonionic alkylphenoxy poly(ethyleneoxy)ethanol surfactant disclosed in U.S. Pat. No. 4,501,793 (Sarada), or a copolymer coating having hydrophilic monomeric units and hydrophobic monomeric units such as an ethylene-vinyl alcohol copolymer disclosed in European Patent Office Publication No. 0 023 459 (Nitadori et al.). Unfortunately, such surfactant treatments to the surfaces of hydrophobic materials may not be permanent due to the washing away of such surface coatings by water or a variety of organic solvents including those used to form the coating on the supporting hydrophobic article. Also, surfactants are commonly known to denature enzymes. See, for example, Molecular Cell Biology, J. Darnell et al. Fds., Scientific American Books, 232, (1986).
Another approach taken in the art is the adsorption of a hydrophilic polymer on a hydrophobic substrate, as disclosed in U.S. Pat. No. 4,794,002 and corresponding European Patent Office Publication 0 221 046 (Henis et al.). A modifying polymer may be adsorbed onto the surfaces of a polysulfone or a polyethersulfone from an aqueous solution of the modifying polymer. But the modifying polymer can be removed with detergent solutions and the like.
Relatively permanent hydrophilic coatings on hydrophobic microporous films have been attempted by further treatment of chemical cross-linking of or ionizing radiation directed against the coating. U.S. Pat. No. 4,346,142 (Lazear) discloses an ionizing radiation process. U.S. Pat. No. 4,776,959 (Kasai et al.) discloses thermally curing a water insoluble vinyl alcohol-vinyl acetate copolymer onto a porous membrane. U.S. Pat. No. 4,753,725 (Linder et al.) discloses semipermeable composite membranes made by reacting PVA/PVA-copolymer films with a monomeric organic compound containing at least two functional groups, a linear or branched polyfunctional oligomer or polymer, and a compound containing cross-linking and ionizable groups. Japanese Publ. No. JP62-14903 (Ohtani et al.) describes using a solution containing a compound having ester side chains and a crosslinking agent to thermally crosslink the ester side chains to hydroxyl or carboxyl reactive sites on the hydrophobic polymer.
Others have attempted to apply hydrophilic poly(vinyl alcohol) directly to the hydrophobic polymer membrane. Japanese Publ. No. JP62-277106 (Ikehara et at.) describes the ionic cross-linking of a poly(vinyl alcohol) on a microporous polymer substrate from a water-soluble poly(vinyl alcohol) polymer containing an inorganic alkaline compound. While poly(vinyl alcohol) has excellent hydrophilicity, processing difficulties are encountered when one attempts to coat hydrophilic poly(vinyl alcohol) directly onto the hydrophobic membrane from a polar or aqueous solution.
Another has attempted to form hollow fiber microporous membranes with poly(vinyl alcohol) chemically bonded to the surfaces of the hollow fiber membrane. U.S. Pat. No. 4,885,086 (Miura) discloses that a hollow fiber membrane is irradiated with ionizing radiations and then reacted with vinyl acetate and hydrolyzed.
The attempts described in the art to provide a hydrophilic poly(vinyl alcohol) coating are based on using atactic poly(vinyl alcohol), which has a low crystallinity content. It is believed that coatings based on atactic poly(vinyl alcohol) are more soluble in a range of solvents and aqueous fluids and consequently the coatings are more readily washed away, particularly when contacted with solvents miscible with the solvents used to bring the hydrophilic material in contact with the hydrophobic membrane.
It is possible to produce poly(vinyl alcohol) which is not atactic. Preparation and the properties of syndiotactic and isotactic poly(vinyl alcohol) have been described in Harris et al., Journal of Polymer Science: Part A-1, Vol. 4, 665-677 (1966), describing the preparation of syndiotactic poly(vinyl alcohol) from poly(vinyl trifluoroacetate) and isotactic poly(vinyl alcohol) from poly(vinyl tert-butyl ether). Further, the production of poly(vinyl trifluoroacetate) as a precursor for syndiotactic poly(vinyl alcohol) has been described in Haas et al., Journal of Polymer Science, Vol. 22, pgs. 291-302 (1956).
Prior uses of such tactic poly(vinyl alcohol) materials have included the preparation of ophthalmic articles, such as contact lenses and coatings for such articles, from non-crosslinked poly(vinyl alcohol) copolymers hydrated to have controlled hydrogel properties and high strength. Co-assigned, related U.S. Pat. Nos. 4,528,325; 4,618,649; 4,693,939; 4,694,037; 4,780,514; 4,840,992; and 4,921,908 (Ofstead) disclose these copolymers and shaped articles, with U.S. Pat. No. 4,693,939 disclosing these copolymers as coatings on articles.
Non-crosslinked crystallized poly(vinyl alcohol) coatings have been disclosed for use with a variety of medical devices. European Patent Publication 0 370 657 (Ofstead) discloses a poly(vinyl alcohol) coating on medical devices (such as catheter guidewires), which is prepared by coating atactic poly(vinyl alcohol) on the device and then annealing the coating to crystallize the poly(vinyl alcohol) to provide a slippery surface.
However, the art of preparing crystallized poly(vinyl alcohol) hydrogel coatings has failed to recognize that in many instances it is desirable to retain the particular geometric configuration of the article being coated. Crystallized poly(vinyl alcohol) which is capable of becoming a hydrogel in the presence of water can disrupt a complex geometric configuration of a supporting structure, such as by blocking the pores of a microporous membrane, if the coating applied to the supporting structure is not carefully controlled.
Filters
The use of porous membrane materials in purification and separation processes is well known. While these porous membrane materials exhibit excellent efficiency in removing fine particulate materials from a fluid, reasonable flow rates of filtrate through the membrane generally require a close match between the polarity of the fluid and the surface energy of the material used to prepare the membrane.
A wide range of materials, including both hydrophobic and hydrophilic materials, have been used to prepare semipermeable membranes. Often, membranes based on hydrophobic materials are stronger than membranes based on hydrophilic membranes, allowing the hydrophobic membranes to be subjected to higher pressures than hydrophilic membranes as a means of improving flow rates through the filter.
The use of porous membrane filters in applications involving aqueous fluids is increasing, as is the need for hydrophilic porous membranes, having physical properties comparable to those realized with many hydrophobic membranes.
Prior attempts to produce hydrophobic membranes having hydrophilic surfaces have generally been unsuccessful, leading to non-permanent treatments, treatments which affected only the outer surface of the membrane and not the surfaces of the inner pores, or treatments which occluded or significantly reduced the pore size of the membrane.